1. Field of the Invention
This invention relates to an improvement in a lens meter capable of displaying distribution mapping of the spherical degree, cylindrical degree, axial angle, and prism degree of a progressive power lens which is a subject lens to be tested (e.g., an eyeglass lens).
2. Description of the Related Art
In recent years, lens meters have been developed. The lens meter is equipped with a light source for generation of a measuring light beam. A progressive power lens (varifocal lens) is set in the optical path of the measuring light beam, and the images of a great number of patterns based on the measuring light beam transmitted through a wide area on the progressive power lens are received, thereby measuring the wide area and performing the image display (mapping display) of the distribution of a spherical degree S (see FIG. 26(a)), a cylindrical degree C (see FIG. 26(b)), an axial angle A (see FIG. 26(c)), and a prism degree Prs (see FIG. 26(d)).
In this kind of lens meter, a subject lens is arranged in an optical path so that the central portion of the subject lens is aligned with the center of measurement of the optical path, and a measuring light beam is transmitted through the wide area of the subject lens, thereby measuring the lens characteristics at each measuring point on the wide area of this subject lens. The lens meter is equipped with imaging/displaying means. The imaging/displaying means displays the lens characteristic information about at least a spherical degree S, a cylindrical degree (astigmatism degree) C, an axial angle A, and a prism degree Prs, based on the lens characteristics at each measuring point on the wide area of this eyeglass lens, and also performs mapping display with the center O of measurement on a screen corresponding to the center of measurement of the optical path as a reference.
When the measurement of an eyeglass lens is performed, there are cases where the lens characteristic values of the spherical degree S, cylindrical degree C, axial angle A, and prism degree Prs of the eyeglass lens are desired to be determined by measuring only a few narrow areas on the eyeglass lens. However, if the lens characteristic values of the spherical degree S, cylindrical degree C, axial angle A, and prism degree Prs of only a desired narrow area on an eyeglass lens are attempted to be obtained with an exclusive lens meter for mapping display, there are the problems that it will take time to obtain each lens characteristic value of the desired narrow area alone and it will be difficult to quickly obtain each lens characteristic value of the desired narrow area alone, because there is a need to compute a great number of measured values obtained by measuring a wide area on the eyeglass lens.
For example, in the case where an eyeglass lens is a lens having substantially the same power all over the surface thereof, such as a general spherical lens or a lens for astigmatism, each lens characteristic value of the spherical lens can be determined by measuring a portion of the spherical lens for astigmatism. In such a case, although it is desirable to measure the lens characteristic values of the spherical degree S, cylindrical degree C, axial angle A, and prism degree Prs of the eyeglass lens by measuring only a desired narrow area of the eyeglass lens, there is the problem that an exclusive lens meter for mapping display will take time for measurement.
Also, where an eyeglass lens as a subject lens is a progressive power lens, there are cases where the measurement is desired to be performed by judging a far point portion (distance portion) M1 and a near point portion (near portion) M2 (see FIG. 26(b)) from the entire data. In order to obtain accurate values of lens characteristics at the far point portion M1 and the near point portion M2 as the measuring points, the progressive power lens is demanded to be placed so that the rear surface of a point to be measured is perpendicular to the axis of a measuring light beam. Hence, in this kind of lens meter, in the state where the rear surface of a subject lens is attached closely to the lens receiver, the subject lens is moved with respect to the measuring light beam so that a desired measuring point on the subject lens is positioned at the center of measurement of the optical path, thereby perpendicularly projecting the measuring light beam onto the measuring point and performing the measurement.
In the aforementioned conventional lens meter capable of mapping display, the lens characteristics at each measuring point on the wide area of a subject lens are measured and image processing is performed based on the lens characteristics at each measuring point, thereby displaying image information on a screen. For this reason, the imaging process takes time, and the movement of a subject lens does not match with an image being displayed on a screen in real time. That is, after the subject lens has been moved, an image where the measuring point and the center of measurement are aligned with each other is displayed late on the screen. Hence, in correspondence with the movement of the subject lens, it is preferable to display in real time an image where the measuring point and the center of measurement are aligned with each other.